Electrocatalysis by design: Effect of the loading level of Au nanoparticles-MnOx nanoparticles binary catalysts on the electrochemical reduction of molecular oxygen

This study concerns the efficient electrochemical reduction of molecular oxygen (O₂), in O₂-saturated 0.1 M KOH solution, to OH⁻ through a four-electron reduction pathway by a novel binary catalyst that is comprised of two kinds of catalysts, i.e., Au nanoparticles (nano-Au) and manganese oxide nanoparticles (nano-MnOx) electrodeposited onto a relatively inert substrate, e.g., glassy carbon (GC) electrode. The nano-Au catalyst is efficiently used for the electro-reduction of O₂ to hydrogen peroxide through a two-electron reduction pathway at a reasonably low overpotential. While the latter (i.e., nano-MnOx) is effectively used for the subsequent catalytic decomposition of the electrogenerated hydrogen peroxide to water and molecular oxygen. The dependence of the electrocatalytic activity of the proposed binary catalysts towards the oxygen reduction on the loading level of both species has been investigated in this paper. This is done aiming at the preparation of a binary catalyst composed of the optimum amounts of both species which supports an apparent four-electron reduction of O₂ at sufficiently low overpotential in replacement of the costly Pt-based electrocatalysts.     

Amperometric detection of hydrogen peroxide at nano-nickel oxide/thionine and celestine blue nanocomposite-modified glassy carbon electrodes

A simple procedure was developed to prepare a glassy carbon (GC) electrode modified with nickel oxide (NiOx) nanoparticles and water-soluble dyes. By immersing the GC/NiOx modified electrode into thionine (TH) or celestine blue (CB) solutions for a short period of time (5–120 s), a thin film of the proposed molecules was immobilized onto the electrode surface. The modified electrodes showed stable and a well-defined redox couples at a wide pH range (2–12), with surface confined characteristics. In comparison to usual methods for the immobilization of dye molecules, such as electropolymerization or adsorption on the surface of preanodized electrodes, the electrochemical reversibility and stability of these modified electrodes have been improved. The surface coverage and heterogeneous electron transfer rate constants (k_s) of thionin and celestin blue immobilized on a NiOx-GC electrode were approximately 3.5 × 10⁻¹⁰ mol cm⁻², 6.12 s⁻¹, 5.9 × 10⁻¹⁰ mol cm⁻² and 6.58 s⁻¹, respectively. The results clearly show the high loading ability of the NiOx nanoparticles and great facilitation of the electron transfer between the immobilized TH, CB and NiOx nanoparticles. The modified electrodes show excellent electrocatalytic activity toward hydrogen peroxide reduction at a reduced overpotential. The catalytic rate constants for hydrogen peroxide reduction at GC/NiOx/CB and GC/NiOx/TH were 7.96 (±0.2) × 10³ M⁻¹ s⁻¹ and 5.5 (±0.2) × 10³ M⁻¹ s⁻¹, respectively. The detection limit, sensitivity and linear concentration range for hydrogen peroxide detection were 1.67 μM, 4.14 nA μM⁻¹ nA μM⁻¹ and 5 μM to 20 mM, and 0.36 μM, 7.62 nA μM⁻¹, and 1 μM to 10 mM for the GC/NiOx/TH and GC/NiOx/CB modified electrodes, respectively. Compared to other modified electrodes, these modified electrodes have many advantages, such as remarkable catalytic activity, good reproducibility, simple preparation procedures and long-term stabilities of signal responses during hydrogen peroxide reduction.     

Effect of composition on the surface and electrocatalytic properties of Ti/IrO x +RhO x electrodes: H2 evolution from acidic solution

IrO _x +RhO _x mixed oxide layers on a Ti support were prepared by thermal decomposition at 450 °C over the whole composition range. The temperature range 450–600 °C was explored for the composition 30 mol% RhO _x . Samples were characterized by means of SEM, XPS, cyclic voltammetry and polarization curves. Their electrocatalytic properties were tested for the H₂ evolution reaction. The following experimental parameters were scrutinized: voltammetric charge, Tafel slope, reaction order (H⁺), electrical resistance of electrocatalysts. The electrocatalytic properties were evaluated at constant potential as a function of temperature as well as of composition. The electrode stability was assessed by comparing CV curves before and after groups of experiments. A reaction mechanism has been proposed. RhO _x is more active than IrO _x , its effect showing up for compositions >30 mol%. In honour of Professor G. Kreysa on the occasion of his 60th birthday. On leave from Institute for Nuclear Research, Pitesti, Romania.     

Effect of the partial replacement of Fe by Ni and/or Mn on the electrocatalytic activity for oxygen evolution of the CoFe2O4 spinel oxide electrode

The study of the electrochemical behaviour of mixed spinel oxide electrodes, obtained by the partial replacement of Fe by Ni and/or Mn in the cobalt ferrite CoFe₂O₄ is presented. The electrodes were prepared by brush painting of iron substrates with a suspension of the respective oxide, prepared by solid-state reaction. The influence of the substituent on the electrodes electrocatalytic activity towards the OER is analysed in terms of the kinetic parameters obtained by steady state measurements and the cationic distribution proposed for the oxides. The data show that the introduction of Ni brings about the presence of Co³⁺ tetrahedrally coordinated in addition to the Co³⁺/Co²⁺ couple in octahedral sites, giving rise to a better electrocatalyst for the OER. In contrast the presence of Mn produces electrodes with lower catalytic activity.     

Investigation of the properties of Ti/[IrO2-Nb2O5] electrodes for simultaneous oxygen evolution and electrochemical ozone production, EOP

A systematic investigation of the influence of Ti/[IrO₂-Nb₂O₅] electrode composition ([IrO₂]=40, 45 and 50 mol%) on electrochemical ozone production (EOP), was conducted in 3.0 mol dm⁻³ H₂SO₄ in the presence and absence of 0.03 mol dm⁻³ KPF₆. “In situ” characterisation revealed all oxide layer presented similar structures, except for the 50 mol% IrO₂ nominal composition which showed a higher porosity/roughness. The introduction of KPF₆ in the electrolyte resulted in an inhibition of the oxygen evolution reaction (OER) at high current densities, improving ozone generation efficiency at i > 0.4 A cm⁻², while reducing overpotential for OER. When normalised for the area, the ozone current efficiency presented a good performance of the system. However, improvement of the electrode service life is necessary in order to support the drastic conditions observed during EOP.     

Tailoring and structure of PtRu nanoparticles supported on functionalized carbon for DMFC applications: New evidence of the hydrous ruthenium oxide phase

The influence of the structure and morphology of PtRu nanoparticles supported on functionalized carbon black has been investigated for CO and methanol electrooxidation in a half-cell and in a DMFC single cell. Carbon black was treated with HNO₃ to obtain an oxidized surface (Vulcan-N), and PtRu nanoparticles supported on Vulcan-N were prepared via impregnation, Bönnemann's method and the sulfito-complex route. Temperature programmed reduction (TPR) measurements evidence the presence of RuO₂·xH₂O phase in the catalyst obtained by the sulfito-complex route. This phase was stabilized by metal–support interaction, whereas alloy characteristics were estimated for PtRu catalyst obtained by impregnation and Bönnemann's method. The nature of the precursor–support interaction, induced by the nature of the functional groups on the carbon surface, affects the structure of the electrocatalyst and subsequent behavior in electroactivity. When synthesized through Bönnemann's method, the surface oxygen-containing groups of the support seem to be unable to stabilize the anhydrous precursors of platinum and ruthenium, yielding crystalline RuO₂. Methanol electrooxidation performance was clearly different in the three catalysts, whereas only a few negligible differences were observed in CO oxidation. The superior performance in DMFC of the catalysts obtained by the sulfito-complex route accounts for both the presence of RuO₂·xH₂O species and the functionalization of carbon black.     

Investigation of the surface properties and the hydrogen evolution reaction, HER, at thermal rhodium oxide electrodes

A systematic investigation was conducted of the surface properties and the HER at electrodes of nominal composition Ti/Rh_xTi_(1−x)O_y prepared by thermal decomposition (T_cal: 500 °C; t_cal: 2 h; O₂ flux: 5 dm³ min⁻¹) from salt precursor solutions dissolved in 6.0 mol dm⁻³ HNO₃. Films were characterized ex situ by SEM, EDX, XPS and XRD and in situ by open circuit potential measurements and CV. The electrochemical behaviour was investigated by CV as function of the anodic, E_λ,a, and cathodic, E_λ,c, switching potentials showing the Rh surface oxidation states strongly depend on these experimental variables. Surface Rh-sites are reduced to metallic rhodium in the cathodic potential region while higher oxidation states (I-III) are formed at more positive potentials (E ≥ 0.5 V/RHE). Hydrogen adsorption and desorption peaks as well as a short double layer charging region are observed at intermediate potential values. The HER was investigated by Tafel coefficients and reaction order with respect to H⁺ as function of nominal Rh-content.     

Ex situ and in situ characterization studies of spin-coated TiO2 film electrodes for the electrochemical ozone production process

An electrode composed of silicon/titanium oxide/platinum/titanium dioxide (Si/TiO_X/Pt/TiO₂) was fabricated by spin-coating TiO₂ multilayers on a Si/TiO_X/Pt substrate and was used in electrochemical ozone production (EOP). EOP was realized when the Si/TiO_X/Pt substrate was completely covered with the TiO₂ film and a current efficiency of 7% was achieved at a low current density of 26.7 mA cm⁻² in 0.01 M HClO₄ at 15 °C. The TiO₂ film was found to be of an anatase-type TiO₂ and that to comprise aperture structures from the X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations. Moreover, the fabricated TiO₂ film was found to be an n-type semiconductor by photoelectrochemical measurements. The high efficiency at a low current density of EOP on the TiO₂ n-type semiconductor was explained to result from the electron transfer through the TiO₂/HClO₄ interface as tunneling current. When the tunneling current passes through a depletion layer of TiO₂, the electrode potential is necessarily high enough to facilitate EOP.     

Reduction and oxidation kinetics of Mn3O4/Mg-ZrO2 oxygen carrier particles for chemical-looping combustion

The kinetics of reduction with methane and oxidation with oxygen of Mn₃O₄ supported on Mg-ZrO₂ prepared by freeze granulation has been investigated. The reactivity experiments were performed in a thermogravimetric analyzer (TGA) using different reacting gas concentrations and temperatures in the range of 1073-1223 K. The oxygen carrier particles showed high reactivity during both reduction and oxidation at all investigated temperatures. An empirical reaction model, which assumes a linear relation between time and conversion, was used to determine the kinetic parameters for reduction and oxidation, with chemical reaction being the main resistance to the reaction. The order of reaction found was 1 with respect to CH₄ and 0.65 with respect to O₂. The activation energy for the reduction reaction was 119 and for the oxidation reaction. The reactivity data and kinetic parameters were used to estimate the solid inventory in the air and fuel reactor of a CLC system. The optimum solid inventory obtained was at a value of ΔX_s=0.4. At these conditions, the recirculation rate of oxygen carrier between air and fuel reactor was per MW of fuel, which could be accomplished in an industrial reactor. The high reactivity of the Mn₃O₄/Mg-ZrO₂ with both methane and oxygen showed that this is a very promising oxygen carrier for CLC.     

Study of carbon-supported IrO2 and RuO2 for use in the hydrogen evolution reaction in a solid polymer electrolyte electrolyzer

Carbon-supported IrO₂ and RuO₂ were prepared using an incipient wetness method and were then calcinated at various temperatures. IrO₂/C and RuO₂/C are less expensive than the conventional Pt/C material and more stable than metal Ni in an acidic electrolyte. Moreover, IrO₂/C and RuO₂/C are not influenced by under potential deposition (UPD) and show lower sensitivity to poisoning by Ni or Fe impurities. The physical properties of IrO₂/C and RuO₂/C were investigated via XRD and TEM. Cyclic voltammograms (CV) and Tafel plots were used to provide information regarding surface redox reaction and electrocatalytic activity. The activity and durability of IrO₂/C and RuO₂/C were studied after prolonged potential cycling between −0.3 and 0.3 V_SCE. After comparison of Tafel plots of Pt/C and IrO₂/C after activation, it was observed that they have similar electrocatalytic activities in a hydrogen evolution reaction (HER). A single cell test with solid polymer electrolyte (SPE) proved that the performance of IrO₂/C (0.5 mg cm⁻²) was similar to that of Pt/C (0.5 mg cm⁻²).     

Reversibility of the l-cysteine/l-cystine redox process at physiological pH on graphite electrodes modified with coenzyme B12 and vitamin B12

We report on the electrocatalytic activity of immobilized coenzyme B₁₂ and vitamin B₁₂ (as aquocobalamin) for the electrooxidation of l-cysteine and their effects on the electrochemical reversibility of the l-cysteine/l-cystine redox couple, a crucial biological system. Cyclic voltammograms of coenzyme B₁₂ adsorbed on a graphite electrode show that upon the reductive elimination of the 5′-deoxyadenosyl group from the cobalt center, at approximately −1.1 V, the electrochemical response of the modified electrode becomes similar to that of aquocobalamin. The electrochemically pretreated coenzyme B₁₂ shows a high electrocatalytic activity for the electro-oxidation of l-cysteine at physiological pH that has never been observed before with the commonly used metallophthalocyanine catalysts. Also, its activity is slightly higher than that exhibited by aquocobalamin.     

Competitiveness of CO2 capture from an industrial solid oxide fuel cell combined heat and power system in the early stage of market introduction

In this article, it was investigated whether potentially low-cost CO₂ capture from SOFC systems could enhance the penetration of SOFC in the energy market in a highly carbon-constrained society in the mid-term future (up to year 2025). The application of 5 MWe SOFC systems for industrial combined heat and power (CHP) generation was considered. For CO₂ capture, oxyfuel combustion of anode off-gas using commercially available technologies was selected. Gas turbine (GT-) CHP plant was considered to be the reference case.Technical results showed that despite the energy penalties due to CO₂ capture and compression, net electrical and heat efficiencies were nearly identical with or without CO₂ capture. This was due to higher heat recovery efficiency by separating SOFC off-gas streams for CO₂ capture. However, CO₂ capture significantly increased the required SOFC and heat exchanger areas.Economic results showed that for above 40-50 $ t⁻¹ CO₂ price, SOFC-CHP systems were more economical when equipped with CO₂ capture. CO₂ capture also enabled SOFC-CHP to compete with GT-CHP at higher cell stack production costs. At zero CO₂ price, cell stack production cost had to be as low as 140 kW⁻¹ for SOFC-CHP to outperform GT-CHP. At 100 $ t⁻¹ CO₂ price, the cell stack production cost requirement raised to 350 $ kW⁻¹. With CO₂ capture, SOFC-CHP still outperformed GT-CHP at a significantly higher cell stack production cost above 900 $ kW⁻¹.     

Semiconductor oxide based electrodes for the label-free electrical detection of DNA hybridization: Comparison between Sb doped SnO2 and CdIn2O4

First results are reported regarding the design, fabrication and operation of a DNA biochip based on a semiconductor oxide electrode that employs label-free electrical detection of the DNA hybridization. The same process of DNA functionalisation, including hydroxylation and silanization steps, was performed on two types of semiconductor oxide: Sb doped SnO₂ and CdIn₂O₄ thin films. These oxide electrodes were laboratory-made films deposited on glass substrates using a chemical vapour deposition method, i.e. the aerosol pyrolysis technique. After having characterized some physico-chemical properties of the bare films, the label-free electrical DNA hybridization detection, without redox couple labelling, was performed using electrochemical impedance spectrometry (EIS) before and after hybridization. On both oxides, over a large frequency range, a significant increase in the impedance modulus was obtained. The increase in the case of CdIn₂O₄ was by a factor of 2.1 ± 0.5 and in the case of Sb doped SnO₂ was by a factor of 1.6 ± 0.1. This phenomenon was especially marked on CdIn₂O₄ thin films, which exhibit a higher sensitivity to the surface event. The DNA hybridization to complementary DNA targets labelled with fluorescent markers was confirmed using fluorescence microscopy.     

Fe-based catalysts for the reduction of oxygen in polymer electrolyte membrane fuel cell conditions: determination of the amount of peroxide released during electroreduction and its influence on the stability of the catalysts

Fe-based catalysts have been prepared by pyrolyzing ClFeTMPP (Cl-Fe tetramethoxyphenyl porphyrin) or Fe acetate adsorbed on PTCDA (perylene tetracarboxylic dianhydride) or on prepyrolyzed PTCDA (p-PTCDA). The catalysts which were already well characterized in terms of active FeN₄/C and FeN₂/C catalytic sites (J. Phys. Chem. B 106 (2002) 8705) are now characterized by RRDE experiments to determine the values of the apparent number of electron transferred (n) and the percentage of peroxide (%H₂O₂) released during the oxygen reduction reaction (ORR) in H₂SO₄ at pH 1. A direct correlation is found between the relative abundance of the FeN₂/C catalytic site in these materials, their catalytic activity and the value of n. The correlation is inverse for %H₂O₂. The best catalysts at their maximum catalytic activity are characterized by n>3.9 and %H₂O₂<5%, equivalent to a value of %H₂O₂ released by a 2 wt.% Pt/C catalyst. It is shown that even low peroxide levels of the order of 5 vol% in H₂SO₄ are able to decompose the catalytic sites releasing iron ions in the H₂SO₄ solution. The loss of catalytic activity correlates directly with the loss of iron ions by these catalysts. All the catalysts have been tested at the cathode of single membrane electrode assemblies (MEAs). The slow decrease in performance in fuel cell stability tests is interpreted as the result of the detrimental effect that has H₂O₂, released during ORR, on the chemical integrity of the nonnoble metal catalytic sites at work at the fuel cell cathodes.     

Localised visualisation of O2 consumption and H2O2 formation by means of SECM for the characterisation of fuel cell catalyst activity

Catalytic reduction of O₂ at metal catalysts either follows a 2-electron transfer pathway to the formation of H₂O₂ or a 4-electron transfer pathway to the formation of H₂O. The branching ratio of this reaction has a significant impact on the applicability of catalyst materials. The previously introduced oxygen competition mode of SECM, which allows high-resolution imaging of local O₂ consumption, has been extended to a potential-pulse sequence enabling the sequential detection of O₂ reduction followed by the detection of local H₂O₂ formation. Thus, not only the local catalytic activity of a catalyst can be visualised but, additionally, the degree of the branching can be estimated. The selectivity of a catalyst candidate can thereby be visualised sequentially to the elucidation of its activity. First results on the properties of electrodeposited Pt and Au test structures are shown demonstrating the feasibility to clearly distinguish catalyst activity and selectivity under variation of the polarisation potential.     

RuO2·xH2O/NiO composites as electrodes for electrochemical capacitors: Effect of the RuO2 content and the thermal treatment on the specific capacitance

RuO₂·xH₂O/NiO composites having RuO₂ contents in the range 0-100 wt.% have been prepared by a co-precipitation method. Structural, microstructural and textural transformations after heating the as-prepared composites at 200 and 600 °C have been followed by X-ray diffraction, scanning electron microscopy (SEM) and nitrogen adsorption/desorption isotherms. At 200 °C the composites are made of micrometric particles in which nanometric crystallites of the two oxides are aggregated. The composites show microporosity (0.02-0.10 cm³/g), mesoporosity (0.07-0.12 cm³/g) and relatively high specific surface area (62-309 m²/g). At 600 °C the composites are fully dehydrated and RuO₂ has crystallized and segregated. Microporosity and mesoporosity as well as specific surface area are strongly decreased. Specific capacitance and specific surface area of the composites heated at 200 and 600 °C have been measured and discussed on the basis of the RuO₂ content. For comparison the specific capacitance and specific surface area of mixtures of NiO and RuO₂·xH₂O (or RuO₂) have been taken as references. The higher specific capacitance of the 200 °C-heated composites compared to the 600 °C-heated ones is due to the higher specific surface area of the former and the higher pseudocapacitance of RuO₂·xH₂O compared to RuO₂. The discussion reported in this work can be applied to other composites such as RuO₂·xH₂O/carbon and RuO₂·xH₂O/other oxides.     

Iron–cobalt mixed oxide nanocatalysts: Heterogeneous peroxymonosulfate activation, cobalt leaching, and ferromagnetic properties for environmental applications

Sulfate radical-based advanced oxidation technologies (SR-AOTs) are attracting considerable attention due to the high oxidizing ability of SRs to degrade organic pollutants in aqueous environments. This study was carried out to respond to current concerns and challenges in SR-AOTs, including (i) need of heterogeneous activation of sulfate salts using transition metal oxides, (ii) nanoscaling of the metal oxide catalysts for high catalytic activity and promising properties with respect to leaching, and (iii) easy removal and recovery of the catalytic materials after their applications for water and wastewater treatments. In this study, we report a novel approach of using Fe–Co mixed oxide nanocatalysts for the heterogeneous activation of peroxymonosulfate (PMS) to generate SRs targeting the decomposition of 2,4-dichlorophenol, and especially focus on some synthesis parameters such as calcination temperature, Fe/Co contents, and TiO₂ support. The physicochemical properties of the catalysts were investigated using porosimetry, XRD, HR-TEM, H₂-TPR, and XPS. Ferromagnetic CoFe₂O₄ composites formed by thermal oxidation of a mixed phase of Fe and Co exhibited significant implications for the efficient and environmentally friendly activation of PMS, including (i) the cobalt species in CoFe₂O₄ are of Co(II), unlike Co₃O₄ showing some detrimental effects of Co(III) on the PMS activation, (ii) CoFe₂O₄ possesses suppressed Co leaching properties due to strong Fe–Co interactions (i.e. Fe–Co linkages), and (iii) Fe–Co catalysts in form of CoFe₂O₄ are easier to recover due to the unique ferromagnetic nature of CoFe₂O₄. In addition, the presence of Fe was found to be beneficial for enriching hydroxyl group content on the Fe–Co catalyst surface, which is believed to facilitate the formation of Co(II)-OH complexes that are vital for heterogeneous PMS activation.     

Polymer membranes for high temperature proton exchange membrane fuel cell: Recent advances and challenges

Proton-exchange membrane fuel cells (PEMFCs) are considered to be a promising technology for efficient power generation in the 21st century. Currently, high temperature proton exchange membrane fuel cells (HT-PEMFC) offer several advantages, such as high proton conductivity, low permeability to fuel, low electro-osmotic drag coefficient, good chemical/thermal stability, good mechanical properties and low cost. Owing to the aforementioned features, high temperature proton exchange membrane fuel cells have been utilized more widely compared to low temperature proton exchange membrane fuel cells, which contain certain limitations, such as carbon monoxide poisoning, heat management, water leaching, etc. This review examines the inspiration for HT-PEMFC development, the technological constraints, and recent advances. Various classes of polymers, such as sulfonated hydrocarbon polymers, acid-base polymers and blend polymers, have been analyzed to fulfill the key requirements of high temperature operation of proton exchange membrane fuel cells (PEMFC). The effect of inorganic additives on the performance of HT-PEMFC has been scrutinized. A detailed discussion of the synthesis of polymer, membrane fabrication and physicochemical characterizations is provided. The proton conductivity and cell performance of the polymeric membranes can be improved by high temperature treatment. The mechanical and water retention properties have shown significant improvement., However, there is scope for further research from the perspective of achieving improvements in certain areas, such as optimizing the thermal and chemical stability of the polymer, acid management, and the integral interface between the electrode and membrane.